The present invention relates to modeforming circuits for antennas. In particular, the present invention relates to a hardware implementation of a modeforming matrix decomposition that efficiently generates mode signals useful for direction finding and beamforming.
Cylindrically symmetric antennas are used in many applications involving, for example, direction finding and beamforming. In these applications, it is often useful to produce the analytic signals referred to as the "modes" of the antenna. In general, a cylindrical antenna with N arms or N input ports has N modes.
In the past, a Butler matrix has provided the circuitry by which the mode signals are produced. The Butler matrix, however, is restricted to antenna designs in which N equals a power of two (e.g., 8, 16, 32 . . . ). Thus, there is a wide range of antenna designs for which the Butler matrix cannot be used (namely, for odd N and even N not a power of 2). Furthermore, the Butler matrix is inefficient in its use of components that implement the phase shifting and signal processing functions that produce the mode signals, particularly as N increases. The complexity, cost, and unreliability of the antenna are correspondingly increased.
Additionally, in many situations, cost considerations may dictate that an antenna include fewer than the number of ports required by the standard Butler matrix. Because of the increasingly large gaps between powers of two (e.g., 16, 32, 64), past antennas requiring a particular performance level (e.g., achieved at N=34) had to bear the increased cost and complexity of using ports that corresponded to the next greatest power of two (e.g., N=64), or implement a design using ports corresponding to a first lesser power of two (e.g., N=32). Thus, compromises in cost and performance were required in the past with standard Butler matrix implementations.
Accordingly, there is a need in the industry for a modeformer circuit that provides reduced cost and complexity, and that may be used with antennas with any even number of arms.